This invention relates to transducer structures and more particularly to a transducer structure employing a relatively rectangular diaphragm having a relatively rectangular active area.
The prior art is replete with a number of transducer configurations. In general, such transducers consist of a bridge arrangement of piezoresistors mounted or integrally formed on a diaphragm. The diaphragm in many structures is fabricated from a thin wafer of silicon and possesses an active area which is conventionally manifested by an aperture in the diaphragm over which the piezoresistive elements are mounted. In this manner, a force applied to the diaphragm causes a deflection of the same; which deflection is attendant with a change in stress on the diaphragm and a consequent change in resistance of the piezoresistive elements which change is indicative of the magnitude of the pressure or force applied to the diaphragm.
Most diaphragms depicted in the prior art are circular in configuration. For examples of transducers employing circular diaphragms, reference is made to U.S. Pat. No. 3,654,579 entitled ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS issued on Apr. 4, 1972 to A. D. Kurtz, et al. and assigned to the assignee herein. Other patents such as U.S. Pat. Nos. 3,748,571, 3,753,196, 3,800,264, 3,739,315 and various other patents too numerous to mention also depict the use of circular diaphragms in conjunction with transducer structures containing various arrangements of piezoresistive devices.
The prior art was also cognizant of the use of other geometric shapes for diaphragms incorporated in transducer structures. As such, certain prior art patents depict rectangular diaphragms employing various grooves and other structures to obtain maximum sensitivity during the force responsive mode. A particular type of diaphragm is depicted in U.S. Pat. No. 4,236,137 entitled SEMICONDUCTOR TRANSDUCERS EMPLOYING FLEXURE FRAMES issued on Nov. 25, 1980 to A. D. Kurtz, et al. and assigned to the Assignee herein. In such a diaphragm structure, a groove frames a central area of the diaphragm with the groove operating as a stress concentrating area to enable sensors mounted and positioned about the groove to provide relatively large output forces upon application of a stress to the diaphragm.
In general, a square or a circular diaphragm is symmetrical about the center point of the diaphragm. The aspect ratio, which is the ratio of the length of the diaphragm to the width of the diaphragm, is 1:1. In such diaphragms, the radial stress is of substantial mangitude and of opposite sign at the center of the diaphragm and at the edge of the diaphragm. In a rectangle, the length to width ratio is greater than 1:1 and for length to width ratios which are greater than 3:1, the ratio of longitudinal to transverse stress approaches theoretical minimum. For the purposes of this invention the longitudinal stress is defined as the stress parallel to the short axis of the diaphragm. In order to utilize a rectangular diaphragm in the most optimum manner, one would desire to take advantage of the maximum longitudinal piezoresistive effect and further, one would desire to substantially reduce or eliminate all transverse stress effects. This is because, for instance in P type silicon the maximum longitudinal coefficient which is in the &lt;111&gt; crystallographic direction is accompanied by a transverse piezoresistive coefficient of 1/2 the magnitude of the longitudinal piezoresistive coefficient and of opposite sign. Thus if such a gage is placed in the center of a circular diaphragm or a square diaphragm a substantial loss in sensitivity occurs. For a rectangular diaphragm however, maximum sensitivity is maintained since there is little transverse stress.
As one can ascertain from the above noted patent literature, the active area of a diaphragm is normally formed by a suitable semiconductor processing technique such as etching an aperture in the diaphragm to define an active area. According to prior art teachings, one incorporates and uses isotropic etching. An isotropic etch is an etchant which etches silicon in all the crystallographic directions at the same rate. The etching materials consist of a mixture of certain acids such as hydroflouric, nitric and acetic acid (HF, HNO.sub.3, CH.sub.3 OOH).
As one can ascertain, by the use of isotropic etching, one forms an active area which possesses sloping sidewalls. This particular configuration can be shown in U.S. Pat. No. 4,204,185 entitled INTEGRAL TRANSDUCER ASSEMBLIES EMPLOYING THIN HOMOGENOUS DIAPHRAGMS issued on May 20, 1980 to A. D. Kurtz, et al. as well as others of the above cited patents. This shape of the sidewalls of the active area is not ideal, one does not provide the most effective edge clamping and furthermore, one does not utilize the material to its optimum degree. The sloping sidewalls serve to reduce the effective active area of the diaphragm and hence, result in material loss, while reducing the effectiveness of the edge clamping and resulting in insufficient area for bonding of the rim or disphragm support.
It is therefore an object of the present invention to provide an approximately rectangular diaphragm having a rectangular aperture with essentially vertical sidewalls defining an active area. It is a further object to provide such a diaphragm having a longitudinal stress, while exhibiting a transverse stress.